It is well known in the separation art that certain crystalline aluminosilicates can be used to separate one hydrocarbon type from another hydrocarbon type. The separation of normal paraffins from branched chain paraffins, for example, can be accomplished by using a type A zeolite which has pore openings from 3 to about 5 angstroms (.ANG.). Such a separation process is disclosed in U.S. Pat. Nos. 2,985,589 to Broughton et al. and 3,201,491 to Stine. These adsorbents allow a separation based on the physical size differences in the molecules by allowing the smaller or normal hydrocarbons to be passed into the cavities within the zeolitic adsorbent, while excluding the larger or branched-chain molecules.
In addition to being used in processes for separating hydrocarbon types, adsorbents comprising type X or Y zeolites have also been employed in processes to separate individual hydrocarbon isomers. In the processes described, for example in U.S. Pat. Nos. 3,626,020 to Neuzil, 3,663,638 to Neuzil, 3,665,046 to de Rosset, 3,668,266 to Chen et al., 3,686,342 to Neuzil et al., 3,700,744 to Berger et al., 3,734,974 to Neuzil, 3,894,109 to Rosback, 3,997,620 to Neuzil and B426,274 to Hedge, particular zeolitic adsorbents are used to separate the para-isomer of bialkyl substituted monocyclic aromatics from the other isomers, particularly para-xylene from other xylene isomers. U.S. Pat. No. 3,707,550 to Stine et al. mentions (as one possibility among a very large group), a silver or silver/potassium exchanged X-zeolite for use in the separation of C.sub.8 aromatic isomers, but only following the removal of the ortho-xylene in a "separation zone" by means which appear to be conventional distillation.
Many of the above patents use benzene, toluene or p-diethylbnnzene as the desorbent. It has been previously recognized that benzene has drawbacks when used as a lower boiling desorbent in contact with the adsorbent (3,686,342 - Neuzil). Toluene suffers in the process for separating isomers of xylene, in recovering desorbent from the extract and raffinate, because the boiling point of the toluene is too close to the isomers being separated. Therefore, an even lower boiling point material, that meets the selectivity requirements for desorbents discussed herein, is desirable. Others of the patents listed above make reference to separation of para-xylene from other isomers of xylene with a zeolite exchanged with barium and potassium in a certain weight ratio. See for example U.S. Pat. Nos. 3,663,638; 3,878,127; 3,878,129; 3,686,342 and 3,558,732. Japanese published Application No. 5155/62, filed Mar. 3, 1960 discloses the separation of aromatic isomers by adsorption on X type zeolites exchanged, for example, with a metal, such as barium, calcium, sodium, etc. Exchange fluids may be benzene, toluene, chlorobenzene, 1-fluoroalkane, .alpha.,.omega.-dihaloalkane, halogen-substituted benzene, halogen-substituted toluene, depending upon the feed mixture, e.g., benzene for xylenes and/or ethylbenzenes, toluene for trimethylbenzenes or ethyltoluenes, chlorobenzene for halogenated aromatic isomer mixture.
U.S. Pat. No. 4,584,424 to Barthomeuf discloses a process for separating ethylbenzene from xylenes by selective adsorption on a beta zeolite. Although para-diethylbenzene is the preferred desorbent for this separation, monohalobenzenes, particularly iodobenzenes, are discussed.